Minimally invasive laparoscopic surgery typically involves making a few small incisions through the outer muscular wall of the body, inserting cannulas through the incisions, adding carbon dioxide or argon gas to inflate the body wall away from the internal organs and thereby create a body cavity, inserting a miniature camera, a light and surgical tools through working channels of the cannulas into the body cavity, and performing the surgical procedure using the surgical tools with the aid of the camera while the gas maintains the inflated condition of the body cavity. Minimally invasive endoscopic surgery typically involves inserting an endoscope through an orifice of the body to gain access to an internal organ such as the lungs, stomach or intestines, and inserting a surgical tool through a working channel of the endoscope. In some circumstances another working channel of the endoscope houses a camera or other optical viewing device and light.
In general, these and other types of minimally invasive surgery require the surgical tool to have an operative device or capability at the end of a relatively long shaft assembly which fits through the working channel, with a handle assembly or other manipulating device connected at the opposite end of the shaft assembly so that the surgeon can manipulate the operative device which contacts the tissue. One common type of operative device is a pair of opposing jaws which are controlled by a jaw movement assembly. The jaws contact, grasp and manipulate tissue.
The jaws, jaw movement assembly and the shaft assembly of the surgical tool must be narrow enough in a transverse dimension to fit through the operating channel of the cannula or the endoscope. The two most common sizes of operating channels are slightly larger than 5 mm across and slightly larger than 10 mm across, thereby allowing a surgical tool having a 5 mm or 10 mm maximum transverse dimension to be inserted through those operating channels. The movement of the jaws is typically accomplished by relative reciprocating longitudinal movement between an outer tubular housing of the shaft assembly and an interior rod within the shaft assembly, and that longitudinal movement is transferred by the jaw movement mechanism into motion which opens the jaws laterally enough to accept and squeeze the tissue between the jaws. The handle assembly at the opposite end of the shaft assembly typically includes a handgrip with a lever which is squeezed to create the relative reciprocating longitudinal movement between the outer shaft housing and the interior shaft rod.
The narrowness of the shaft assembly and jaw movement mechanism imposes structural and functional limits on the amount of force which can be transmitted to the jaws. The jaw movement mechanism converts the relative reciprocating longitudinal movement of the shaft housing and the shaft rod into transverse movement of the jaws. The mechanical geometry involved in this conversion usually diminishes the force available from the jaws compared to the force available due to the relative movement of the shaft housing and shaft rod. Some of the mechanical force applied to the jaw movement mechanism is lost due to frictional resistance between moving parts of the jaw movement mechanism. The narrowness of the shaft assembly, jaw movement mechanism and jaws also limits the degree and extent of movement of the jaws. Because the jaws must not extend any wider than the operating channel when the jaws are closed, the jaw movement mechanism must establish a movement geometry which causes the jaws to open or separate sufficiently to grasp a desired amount of tissue, yet close to a position where the jaws are positioned very close to one another.
One advantageous type of jaw movement mechanism causes one or both of the jaws to separate from a closed position in a parallel manner until a certain degree of separation is achieved, and then moves one or both of the jaws in a pivotal manner similar to scissors to open the distal tip ends wider than the proximal ends. The completely open pivotal action allows more tissue to be located between the jaws and makes it easier for the surgeon to contact tissue which is to be grasped between the jaws. The closing parallel movement is advantageous in many surgical procedures to apply uniform pressure across the tissue which is grasped between the jaws.
Applying uniform pressure as a result of parallel closing jaw movement is particularly useful for those types of surgical tools which join, weld or fuse tissue together. In these types of instruments, heater elements or electrodes are attached to or incorporated in the jaws to allow the application of thermal energy to the tissue grasped by and compressed between the jaws. Fusion of the tissue pieces results from the simultaneous application of pressure and heat. The heat comes directly from the heater elements or is created by current flowing in the tissue between the electrodes on the jaws. Tissue fusion is typically performed on vessels to block the fluid or blood flow carried by the vessels. Achieving good tissue fusion is substantially enhanced if the vessel walls are compressed together with uniform pressure. Uniform pressure is obtained from the parallel closing movement of the jaws, and the uniform pressure is more likely to achieve an effective, leak-proof seal. Additionally, the parallel withdrawal of the jaws for limited range of motion after the tissue fusion has occurred is less likely to damage the seal which has been formed. The parallel withdrawal movement avoids a mechanical shearing action on the tissue which would occur if the jaws opened by a pivoting separation movement.
A common type of jaw movement mechanism which is capable of transferring longitudinal relative reciprocating motion into parallel movement of the jaws when closely approximated and into pivoting movement of the jaws when further separated involves two sets of pins and slots. Each of the two slots has a predetermined geometric and angular shape, and each of the two pins moves within its own slot. The slots may be formed in a distal end of the shaft housing, or in an attachment on the shaft housing, or in one or both of the jaws. The pins may likewise be attached to the distal end of the shaft housing, or to an attachment on the shaft housing, or in one or both of the jaws. The relative longitudinal reciprocating motion of the shaft housing and shaft rod moves the pins in relative position within the slots, and the geometry of the slots in relation to the position of the pins in those slots cause the jaws to move with a pivoting motion when the jaws are significantly separated and to move toward one another with parallel motion when the jaws become closer to one another. In some types of jaw movement mechanisms, only one pin and slot combination is used, coupled with some other form of mechanical movement-inducing device.
A drawback of the pin and slot type of jaw movement mechanism is that a significant amount of the force from relative longitudinal reciprocating movement of the shaft housing and the shaft rod is lost due to sliding frictional contact of the pins with the slots. The geometric orientation of the slots involves portions which are inclined with respect to one another, and the movement of the pin along the inclined portion of the slot generates substantial friction and reduces the amount of force available to move the jaws. This frictional sliding contact reduces the mechanical efficiency of the jaw movement mechanism. The amount of efficiency lost by the frictional engagement of the pins with the slots can reduce the amount of force available on the jaws to the point where the pressure is insufficient for tissue sealing.